Improved energy density for rechargeable Li-ion batteries requires use of high-capacity and high-voltage cathode materials, but charging to voltages approaching 4.5-5V invariably causes rapid loss of capacity with cycling. This degradation is attributed to several mechanisms, including oxygen loss, transition metal dissolution, lattice or particle instability, or reactions with the electrolyte or impurities. One promising solution is to coat the cathode particle surface with a protective material, such as a metal oxide. At this time, it remains unclear why such metal oxide coatings improve the high-voltage cycling behavior and whether film uniformity is critical.
Generally coatings for cathode materials are stable compounds that do not contain lithium, such as AlPO4, ZrO2, Al2O3, ZnO, and Bi2O3. Coatings are typically applied to cathode powders by solution or sot gel coating. These materials then need to be heat treated at elevated temperature to decompose the precursor and form the oxide coating. The amount and morphology of the coating is not well characterized. In some cases the coating is clearly not uniform. In many cases, the coating adds substantially to the mass.